1. Field of the Invention
The present invention generally relates to printers operated by host computers, and more particularly to a method and apparatus for controlling or altering the energy supplied to a printer print head in accordance with the type of recording media being used by the printer, especially for multiple paper-type POS printers. The invention further relates to a printer control structure that selectively adjusts the length of time driving voltage is applied to a print head in association with the type of recording paper being used.
2. Related Technological Art
In many printers of the computer and point-of-sale (POS) type, a print head is used that employs a series of small electromagnetic coils to operate a corresponding series of print pins, which are used to form an output image by striking an ink ribbon. The print head coils receive voltage signals that impart a driving voltage to the head to operate the coils and activate the pins. Other types of ink transfer structures are also sometimes employed which also rely on certain voltage input signals for activation.
In any case, the print head driving voltage is normally supplied from a power source or supply such as an AC power source that uses a switching regulator, a series regulator, or other type of DC stabilized control. Such power sources have a tolerance of about plus or minus ten percent (.+-.10%) variation from a rated voltage value so that the actual driving voltage may differ from printer to printer. Furthermore, these stabilized power sources still have internal impedance or resistance, and the output voltage changes with changes in the load applied to the output. In addition, because of the impedance or resistance characteristics of any wiring connecting the power source to any print head drive circuit, or the coils, the driving voltage supplied to the print head drive is often not fixed but fluctuates to some extent. Therefore, assuming a relatively constant length of time for the application of the driving voltage to the print head, the resulting energy delivered to the print head does not remain constant but varies among various printers (static fluctuations) causing a print quality tolerance range or variation among the printers and is not constant within the individual printers (dynamic fluctuations), resulting in a certain level of unevenness in the printing output quality.
In order to make the energy imparted to the print head constant or more time invariant, the output of the power supply is controlled in a manner such that the length of time the voltage is applied is also varied to form a constant energy product. That is, when the voltage decreases to a lower energy per unit time value, the print head is driven for a longer period of time to provide the same net energy input. Conversely, the print head driving time is decreased when the driving voltage increases. A graphical representation of the relationship between print head driving voltage and driving time is presented in FIG. 8. From FIG. 8, it is readily apparent that the relationship between these two parameters has a roughly 1-to-1 correlation, as indicated by values for the driving period T.sub.1 with respect to the driving voltage V.sub.1 on the scale of pin driving voltage V.sub.p. By controlling the print head so that the print head driving time is more suited to the print head driving voltage, printing quality is stabilized.
Unfortunately, this simplistic type of stabilization does not apply as well to printers that are capable of accommodating multiple types of print media or paper. That is, when the thickness and material of the recording paper being used varies, it is difficult to consistently obtain high print quality using the above method of matching driving voltage to driving time with a 1-to-1 or linear relationship. When a single sheet of paper is used in the printer, the print head driving time can be relatively short but when multiple layers of paper are used, as for multi-part forms and copies, the overall thickness of the print media is increased which also increases the energy required and the driving time must be increased.
Using the current technique, however, the same print head driving time is used for printing single paper sheets as is used for multiple sheets at a given, the same, print head driving voltage. This results in a driving time longer than necessary for single sheet paper, which increases the printing noise, overheats the print head, etc. On the other hand, if a driving time used for multi-sheet media is the same as that typically used for single sheet media, the ability to make copies is severely degraded.
In order to solve the energy transfer problem created by varying the media type, some printers are being developed which control the length of time voltage or power is applied to the print head according to the relative thickness of the recording paper being used. Examples of such printers are found in Japanese Laid-Open Patent Publications 53-118315 (Pat. No. 59-33118), 03-23953, and 03-93549.
The printer shown in the 53-118315 publication is provided with a detector for sensing the presence of a paper thickness indicator which is affixed to the recording paper. A print head activation time signal, typically a coil or magnet energizing time indicator signal, is generated which corresponds to the paper thickness and varies according to the thickness indicator sensed by the detector. This signal is used to automatically determine the thickness of the recording paper and control the energizing current, applied voltage, and driving time for the print head in accordance with the sensed paper thickness.
The printer shown in the 03-23953 publication, has a special motor which is used to adjust the gap between the printer platen and the print head, and a gap sensor to determine the current size of this gap. By measuring the difference in gap size between the point when no recording paper is present and when recording paper is present on the platen, a relative thickness for the recording paper is determined. From this determination of paper thickness, the length of time for driving the print head is selected.
As in the case of the printer shown in the 03-23953 publication, the printer shown in the 03-93549 publication has a motor which is used to adjust the platen gap and a sensor for determining the maximum physical gap size for the printer. Then, by measuring the distance by which the print head is moved from the maximum open gap configuration until it contacts the recording paper, such as by monitoring the rotation of the motor, the thickness of the recording paper can be detected. Once the thickness is measured, the length of time over which voltage is applied to the print head, as in applying power to coils used to drive pins, is adjusted accordingly.
However, the printers disclosed in these examples require a special mechanism to detect the thickness of the recording paper. That is, in the first example, a paper thickness indicator affixed to the recording paper along with an associated detector are each required. For the printers in the second and third examples above, both a platen gap adjustment motor and an associated gap sensor are required. Adding these special components or devices decreases the general usability of the recording paper. At the same time, reliability and ease of assembly are degraded due to a more complex mechanical configuration for the printers, and production costs are increased, which also hinders general acceptance in the marketplace.
Furthermore, while these printers can accommodate various paper thicknesses, they do not readily accommodate variations in other characteristics, such as variations in paper quality, that can effect printing. For example, even though a single sheet of paper and a multi-layered form have the same overall thickness, they require different print head energies or driving times, due to other surface characteristics, resiliency, etc., that differs between these media. The current state of the art printers cannot meet this need.
What is needed then, is a printer mechanism and a method of operation in which the print head driving time period is altered in response to fluctuations in the magnitude of the print head driving voltage. It would also be appropriate if the period of operation for the print head was optimized to readily accommodate differences in paper thickness and quality.